If you read my post Restoring Standby Database, you know that one of the problems I had was that the new control file refused to open the database with resetlogs even though the database was consistent.
Even after years of working with something, you can always learn new stuff. Today I tried to create a standby database using the duplicate command. When you duplicate a database you need to connect to both instances (primary as target and standby as auxiliary) using SQL*Net (and not “/”). Since the standby is in nomount, the listener blocks connections to it, so when trying to connect to it using the listener we get “ORA-12528: TNS:listener: all appropriate instances are blocking new connections”.